Defend against bugs in SelectorFlagsMap

* Make `SelectorFlagsMap::apply_flags` private so it's easier to ensure
  it is called from the correct thread.

* Clear the cache and the map at the same time; otherwise it leaves
  `SelectorFlagsMap` in an inconsistent state.

components/style/context.rs

@@ -553,8 +553,9 @@ impl<E: TElement> SelectorFlagsMap<E> {
     }
 
     /// Applies the flags. Must be called on the main thread.
-    pub fn apply_flags(&mut self) {
+    fn apply_flags(&mut self) {
         debug_assert!(thread_state::get() == ThreadState::LAYOUT);
+        self.cache.evict_all();
         for (el, flags) in self.map.drain() {
             unsafe { el.set_selector_flags(flags); }
         }